A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor manufactured using crystalline silicon has high field effect mobility, but a crystallization step such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate.
In contrast, attention has been drawn on a technique by which a thin film transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 each disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O based oxide semiconductor formed into an oxide semiconductor film and is used for a switching element or the like of an image display device.